Lithium secondary batteries, which have been in the spotlight as a power source of portable and small electronic devices, may exhibit high discharge voltages that are two times or more than those of batteries using a typical alkaline aqueous solution by using an organic electrolyte solution. Thus, the lithium secondary batteries exhibit high energy density.
Oxides formed of lithium and transition metal which have a structure capable of intercalating lithium, such as LiCoO2, LiMn2O4, and LiNi1−xCoxO2 (0<x<1), have been mainly used as a cathode active material of a lithium secondary battery, and various types of carbon-based materials including artificial graphite, natural graphite, and hard carbon, which are capable of intercalating and deintercalating lithium, have been used as an anode active material.
Graphite is mainly used as an anode material of the lithium secondary battery. However, graphite has a low capacity per unit mass of 372 mAh/g and a high-capacity lithium secondary battery may be difficult to be prepared by using graphite.
As an anode material exhibiting higher capacity than graphite, a material forming an intermetallic compound with lithium, such as silicon, tin, and an oxide thereof, may be promising. However, volumes of the above materials may expand because crystal structures thereof may be changed when absorbing and storing lithium. When silicon absorbs and stores the maximum amount of lithium, the silicon may be transformed into Li4.4Si and the volume of Li4.4Si may expand due to charging. With respect to the rate of increase in volume due to the charging, the volume may expand up to about 4.12 times the volume of the silicon before the volume expansion.
Therefore, a significant amount of research into an increase in the capacity of an anode material, such as silicon, i.e., a decrease in a volume expansion coefficient by alloying of silicon, has been conducted. However, since a metal, such as silicon (Si), tin (Sn), and aluminum (Al), is alloyed with lithium during charge and discharge, volume expansion and contraction may occur. Thus, cycle characteristics of the battery may degrade.
Since higher capacity characteristics than those of a carbon-based material may be obtained and volume expansion may be suppressed in comparison to Si when a non-carbon-based material, such as SiO, is used, a great deal of research into the non-carbon-based material has been conducted. However, since a reaction between lithium (Li) and oxygen (O) forming by-products is represented by an irreversible reaction, initial efficiency may be decreased.
Therefore, a significant amount of research into SiO has been conducted to improve the above-described limitations. For example, Korean Patent Application No. 2012-7011002 discloses an anode active material for a lithium ion secondary battery using SiOx. However, it has limitations in that charge and discharge cycle characteristics may not be sufficiently improved and an x value in SiO, may be difficult to be controlled by a conventional synthesis method.